Options Sound: silent not working?

Recently i tried to add The silent options to some zombies and it seems not to work..

I dont know if its Just a syntax problem or what, because i tried preventrandomequipment too and after some spawning it is also not working .-.

Could someone just test and tell me if its working? And if it is post The code for it? I am doing some ambient mobs and had severall tries with this command